Nitrophenols have been of considerable interest because they are used as intermediates in the synthesis of some organophosphorus pesticides, explosives, leather coloring agents, dyes, and some pharmaceuticals products. However, nitrophenols are known as toxic, inhibitory, and biorefractory organic compounds. Releasing these compounds into the environment and streams as the effluent of industrial wastewater has toxic effects on the ecosystem and human health due to their persistence in the environment [1-6]. Therefore, the elimination of nitrophenol is gaining extensive attention in the quest for efficient and benign conversion processes in the area of environmental management. The conventional approaches to the removal of dyes from wastewater are adsorption, oxidation, microbiological, photocatalysis, and so on [7-10]. Among them, photocatalytic decomposition of organic compounds in wastewater has attracted a great deal of attention due to the viewpoint of green chemistry [11-17]. The TiO2 is one of the most effective photocatalysts for its photostability, generally biologically and chemically inert but the critical drawback is the large bandgap which limited its application [18-22]. Therefore, for solving this problem, using photosensitizer agents including metalloporphyrins is a good manner to overcome this disadvantage [1, 23].
Also, metalloporphyrin derivatives especially the strong anchoring groups (e.g. hydroxyl, carboxyl, and sulfonic acid groups) can form a strong covalent band facilitating the charge-transfer interaction and maximize the charge-injection efficiency .
However, it is a well-known fact that heterogenization of metal complexes on solids supports show several advantages including the long lifetime of catalyst, facile recovery, and easy separation of the catalyst [24-29]. Several studies on the synthesis and photocatalytic activity of encapsulated metal complexes such as iron (II) bipyridine supported on Na-Y zeolite have been reported the degradation of malachite green . Fe-exchanged Y zeolite as a catalyst was used for degradation of the reactive dyes by catalytic wet hydrogen peroxide oxidation [31, 32]. photodegradation of 2,4-dichlorophenol and organic pollutant using metallophethalocyanine and supported substituted metallophethalocyanine on MCM-41 under light irradiation was reported [19, 33-35].
In all of AOP’s processes, oxidation reagent requires for completion of the degradation of 4-NP. AOPs based on H2O2 produce highly reactive species .OH and they degrade a broad range of organic materials. However, in the high concentration of H2O2, it leads to the formation of HO2. radical which in turn, acts as a scavenger of .OH radicals and consequently it reduces the efficiency of photodegradation . To overcome these disadvantages and to inhibition of the usage of oxidant, we wish to report the synthesis and physicochemical characterization of CeTPP(OH)n/TiO2/NaY (n=0-4) nanocomposite. The photosensitized oxidation of 4-nitrophenol using this heterogeneous system is reported on a lab-scale for the first time. Besides, substituted metalloporphyrin with several stoichiometric ratios is synthesized. Moreover, by using substituted metalloporphyrin with OH group in a peripheral position, the photodegradation of 4-NP was performed without H2O2. Furthermore, by increasing the number of substituted metalloporphyrin, the efficiency of photodegradation is enhanced.
MATERIALS AND METHODS
All materials were of the commercial reagent grade and were purchased from Sigma-Aldrich, Merck, and Fluka, Mojallaly industrial Co, Loba chemie, and Arman sina. Pyrrole and benzaldehyde were distilled before use.
Preparation of Ce(OH)nTPP/TiO2/NaY
H2(OH)nTPP was prepared using freshly distilled pyrrole (4 mmol,) and benzaldehyde-para hydroxybenzaldehyde (with various ratios from 0-4 mmol) in propionic acid as solvent.
Cerium (IV) tetraphenylporphyrin was prepared according to our previous works ,. Ce(OH)nTPP (n=0-4) (n indicates the number of OH hydroxyl groups on the peripheral ring) was synthesized inside the nanocage of NaY zeolite by hydrothermal treatment . The aluminosilicate gel was prepared by stirring colloidal silica (2.3 g), sodium hydroxide (3.1 g), sodium aluminate (1.6 g), and H2O (40 mL). After then, Ce(OH)nTPP (0.02 g, 3.6×10−4 mol) was added. The crystallization occurred at 95°C under static conditions in a stainless steel bomb (250 mL) for 48 h. The adsorbed complexes on the external surface of the zeolite were removed through Soxhlet extractions with di-Chloromethane, ethanol, and acetone. The final product was dried at 80°C for 24 h.
To encaging TiO2 on zeolite structure, 1 g CeTPP(OH)n/NaY was added to the suspension containing 0.16 g TiO2 in 50 mL distilled water with stirring for 36 h. Then, the solvent was evaporated under vacuum and dried at 100 °C for 12 h. The obtained materials were calcinated at 523 K for 5 h.
General procedure for the photodegradation of 4-Nitrophenol
For this purpose, a 0.02 g catalyst was added to the solution of 50 mL of 4-Nitrophenol (4×10-5 M) and then stirred at the dark to obtain adsorption/desorption equilibrium to eliminate the error due to any initial adsorption effect. After that, the irradiation experiments were carried out in the presence of H2O2 in a photocatalyst reactor. In another typical procedure, the photodegradation reaction was performed in the absence of H2O2 to investigate the OH effect in the peripheral position of the porphyrin ring on the photodegradation process. The suspension was magnetically stirred during irradiation. Also, to monitor the 4-Nitrophenol degradation process, about 3 mL of the suspension was withdrawn and the photocatalysts were separated from the suspension by filtration. Finally, the reaction progress was monitored by its characteristic absorption band at 317 nm using a double-beam UV-Visible spectrometer.
Chemical oxygen demand
Chemical oxygen demand (COD) is a measure of the amount of the required oxygen for decomposing organic matter. This indicates an aggregate measure of the organic compounds that can be oxidized to inorganic (mineral) materials at the end of the reaction. Therefore, it was used to evaluate the degree of mineralisation that had occurred during the photocatalytic process. The COD at the beginning and the end of the experimental runs was determined by volumetric titration methodology .
RESULTS AND DISCUSSION
Preparation and characterization of CeTPP(OH)n/TiO2/NaY
The synthesis of zeolite-encapsulated (OH)n-substituted cerium (IV) tetraphenylporphyrin (ZECTPP(OH)n) is carried out by the zeolite synthesis method. In this method, the anionic template was synthesized around the cationic guest using an electrostatic interaction. Clearly, this method is a clean synthesis way since there isn’t any free metal ion, free ligand, and other impurities in the zeolite cages, and therefore, the characterization of catalyst seems easy. The crude material was treated with Soxhlet extraction with ethanol, dichloromethane, and acetone to remove the excess ligand and external surface adsorbed complexes. Encaging of TiO2 in zeolite cage performed by impregnation method. The Ce and Ti content of the photocatalyst is determined by ICP methods (Table 1). The results show that TiO2 amounts in all of the samples are similar that proves the TiO2/catalyst ratio is constant.
The XRD patterns of the photocatalysts in Fig. 1 indicate that all synthesized catalysts have crystallinity almost identical to that of the parent NaY zeolite with ICDD file no. 083-1084. The presence of the characteristic peaks at 2θ=25, 37, and 46° related to the anatase phase of TiO2, also proves the insertion of TiO2 into the pores of the zeolite. However, the peak in the region of 2θ=12° can be related to metalloporphyrin species. In addition, the intensity of the main peak in the zeolite structure remained unchanged, which indicating the preservation of the framework and crystallinity of zeolite upon encapsulation of cerium (IV) porphyrin into its supercages. It confirmed that the catalyst distributed as fine particles in the zeolite pores. Moreover, by functionalizing the porphyrin rings, the XRD pattern preserved except for the slight change that proved the presence of OH group in peripheral position is without any steric spatial. However, reflections in the regions of (2 2 0), (3 1 1), and (3 3 1) are related to the location of cations. Upon the encapsulation of complexes in the cages of zeolite, the order of relative peak intensities changes from (3 3 1) > (2 2 0) > (3 1 1) to (3 3 1) > (3 1 1) > (2 2 0) which is confirmed the substitution of Na+ ions by Ce4+ ions. Then, a rearrangement occurs during the complexation .
The FT-IR spectra of NaY, CeTPP/TiO2/NaY and CeTPP(OH)n/TiO2/NaY zeolite are shown in Fig. 2. No shift observes in the position of the zeolite lattice bands in the encapsulated metalloporphyrins. However, bands in accordance with cerium porphyrin observe. In these spectrums, the bands in the range 450–1200 cm−1 are attributed to the strong zeolite lattice vibration, and bands in 3200–3600 cm−1 and 1636 cm-1 are due to lattice water molecules and the surface hydroxyl groups. Band in region 2900 cm−1 and 1150-1500 cm-1 is in accordance to C-H and C-C, and C-N vibration respectively indicates the presence of metallocomplexes into zeolite cage ,. Also, a peak in the region 2922 cm-1 is related to O-H vibration in the peripheral porphyrin situation which its intensity is increased with enhancing of substituted ring number (Fig. 2b, c, d, e). Besides, peaks in the 800-890 cm-1 are related to the M–O–M vibrations (where M=Si, Al, Ce, and Ti) . Meanwhile, absorption bands in the region of 860-920 cm-1 can be related to the TiO2 species stretching vibration that masked by zeolite structures .
Fig. 3 shows the field emission scanning electron micrograph (FESEM) of the CeTPP(OH)n/TiO2/NaY zeolite with the cubo-octahedral units. Based on encapsulation of metallocomplex, the morphology of zeolite remained unchanged confirmed in the template synthesis method, metallocomplexes act as a template, and the zeolite structure formed around it. Moreover, the presence of some species in nanometer size proved the insertion of TiO2 into the pores of zeolite (Fig. S1).
Further, the FESEM-EDX spectrum of CeTPP(OH)n/TiO2/NaY nanocomposite showed the presence of a component of the photocatalyst. Thus, it demonstrated the formation of metallocomplexes into zeolite cages (Fig. S2).
The transmission electron micrograph (TEM) of CeTeHPP / NaY / TiO2 is shown in Fig. 4, showing uniform Cubo-octahedral units that encompassed the supercages.
Investigation of photocatalytic activity of CeTPP/TiO2/NaY
To detect the photocatalytic activity of CeTPP/TiO2/NaY, degradation of 4-Nitrophenol is tested as a function of different experimental parameters are presented in Figs. 5(a-d).
The effect of catalyst amount on the removal of 4-Nitrophenol was studied by varying the catalyst amount from 0.005 to 0.03 g. The results collected in Fig.5a. We found that with an increase amount of the catalyst, degradation of 4-Nitrophenol was enhanced because of an increase in the absorption of dye molecules. However, the optimum degradation was obtained with a 0.01 g catalyst. At high catalyst loading, aggregation of species increases, in turn, reduces the catalytic activity of the photocatalyst because of the turbidity of the solution , .
To obtain the optimum ratio, a variety of TiO2/catalyst ratio including 1:6, 1:8, 1:10, 1:15 tested in the photodegradation of dye molecules. The results showed that the maximum photocatalytic activity was obtained with 1:6 ratios. It seems with increasing TiO2 amount, the reduction between catalyst and dye occurred which followed by decreasing in the interfacial area between the reaction solution and the photocatalyst. Consequently, the degradation of 4-NP reduced  (Fig. 5b). To study the catalytic activity of CeTPP/TiO2/NaY and the role of H2O2 in catalytic oxidation of dye, dye degradation experiments with different values of H2O2 were carried out. It suggests that a system with 1 mL H2O2 gives maximum removal of 4-NP. So, upon increasing in H2O2 value, the removal of dye decreases. It is likely due to the higher concentration of H2O2 since it acts as a scavenger and then, photocatalytic activity is disrupted. It seems that in the higher concentration of H2O2, the main part of .OH radicals have been consumed by H2O2 which was followed by the formation of less reactive HO2. radical . Finally, photodegradation of 4-NP reduces (Fig. S3). Under the optimum conditions, photodegradation of 4-NP is investigated in the presence of CeTPP(OH)n/TiO2/NaY catalyst under optimum conditions. The obtained results are displayed in Fig. 5c. The reaction is completed within one hour, and after that, more removal of dye did not occur.
To investigate and compare the effect of OH group anchored to porphyrin ring on photodegradation process, the reaction is performed in the absence of H2O2 with the various ratios of substituted metalloporphyrin and the presence of 1 mL H2O2 with net metalloporphyrin. The results revealed that by increasing OH-substituted porphyrin number, photodegradation yield enhanced so that the photodegradation yield in the presence of CeTPP(OH)4/TiO2/NaY without H2O2 become better than CeTPP/TiO2/NaY with 1 mL H2O2. It seems that OH in the peripheral position of the porphyrin ring can be broken as hemolytic and act as .OH radical obtained from H2O2 which it is the driving force for AOP’s processes (Fig. 5d).
Moreover, Table. 2 shows the efficiency of our catalytic system with the efficiency of some other catalysts in the photodegradation of 4-NP. It is clear that our catalyst is a mild, effective with high efficiency system.
Mineralization of 4-NP upon the photodegradation reactions is established by COD analysis. It is observed that the obtained solution after completing the photodegradation reaction shows a significant COD removal (53.2%) for CeTPP/TiO2/NaY and 57% for CeTPP(OH)4/TiO2/NaY after 60 min irradiation. Thus it proved that a part of the organic material is mineralized. This, in turn, indicates that the mineralization of 4-NP is followed by the formation of simple inorganic materials such as carbon dioxide and water during photodegradation.
In this study, the preparation of CeTPP(OH)n/TiO2/NaY catalyst was performed using zeolite synthesis and impregnation methods respectively. XRD studies show that upon the incorporation of the metal complex, crystallinity of the zeolite matrix remains unchanged. In addition, the appearance of the peak in the 2θ=25 indication of TiO2 proved the insertion of TiO2 in nanopores of zeolite. Also, the catalyst displays an efficient photoactivity for the degradation 4-Nitrophenol in the presence of H2O2 under UV illumination. Interestingly, the best results are obtained within 1 h for a concentration of 5×10-5 of 4-NP, 1:6 catalyst/TiO2, the amount of catalyst loading about 0.01 g and, 1 mL H2O2 under UV irradiation. In addition, the photodegradation process is performed by OH-substituted porphyrin encapsulated into zeolite NaY, and TiO2 in the absence of H2O2 and the results were in the better yield.
We are grateful from Maragheh University for support this work.
CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.